Marine Macrophytes as Carbon Sinks: Comparison Between Seagrasses and the Non-native Alga Halimeda incrassata in the Western Mediterranean (Mallorca)

Lukas Marx,Marlene Wesselmann,Carlos Morell,Iris Eline Hendriks,Susana Flecha

doi:10.3389/fmars.2021.746379

Abstract

Seagrass species play a critical role in the mitigation of climate change by acting as valuable carbon sinks and storage sites. Another important ecosystem service of this coastal vegetation is nutrient removal. However, coastal ecosystems are under increasing pressure of global warming and associated establishment of invasive species. To elucidate the respective contributions of seagrass species Posidonia oceanica and Cymodocea nodosa and the non-native macroalga Halimeda incrassata as primary producers and nutrient sinks in coastal habitats we conducted in-situ incubations in the North-western Mediterranean Sea. Measured metabolic activity and nutrient removal as well as calcification rates in these habitats over a 24 h period in spring and summer confirmed that the endemic seagrass P. oceanica represents a valuable ecosystem with high O2 production and considerable carbon capture. The documented regression of P. oceanica meadows with higher temperatures and decline in autotrophy as measured here causes concern for the continuity of ecosystem services rendered by this habitat throughout the Mediterranean Sea with progressing climate warming. In contrast, the enhanced performance of C. nodosa and the calcifying alga H. incrassata with increasing temperatures, under expected rates of future warming is uncertain to mitigate loss of productivity in case of a potential shift in marine vegetation. This could ultimately lead to a decline in ecosystem services, decreased carbon storage and mitigation of climate change. Furthermore, this study provides a first estimate for the growth rate of H. incrassata in the Mediterranean Sea, supporting evidence for the mechanism of its rapid extension.

Highlights

Ever since the 18th century, humanity has caused raised and still increasing emission of greenhouse gases (GHGs), mainly carbon dioxide (CO2) with annual averages of 410 ppm in 2019 (IPCC, 2021) and predicted CO2 levels four times higher compared to pre-industrial (Wigley, 1983) levels (IPCC, 2021)
The time period investigated was from early spring, when temperatures rose and H. incrassata was observed to enter its active state, to summer, when water temperatures approached annual temperature maxima in the Mediterranean Sea, spanning across the crucial period for the success of colonization of H. incrassata
community respiration (CR) [F(2,2) = 28.24] and gross primary production (GPP) [F(2,2) = 35.45] are significantly higher than for C. nodosa and H. incrassata (p < 0.05), Net community production (NCP) showed no significant difference between species (p = 0.445)

Summary

Introduction

Ever since the 18th century, humanity has caused raised and still increasing emission of greenhouse gases (GHGs), mainly carbon dioxide (CO2) with annual averages of 410 ppm in 2019 (IPCC, 2021) and predicted CO2 levels four times higher compared to pre-industrial (Wigley, 1983) levels (IPCC, 2021). The ocean plays a relevant role mitigating the CO2 rise in the atmosphere through the absorption of around a third of the anthropogenic emissions (Gruber et al, 2019) This CO2 uptake impacts the chemical balance of seawater by increasing bicarbonate ion concentration (HCO3−) and reducing the availability of carbonate ions (CO23−) by 60% (Feely et al, 2004). This shift in water chemistry results in a decrease of seawater pH, a process referred to as Ocean Acidification (OA; Doney et al, 2009). Ocean acidification is a global phenomenon, impacting calcification and metabolism of organisms, which has been documented in many marine regions (Kroeker et al, 2013)

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